It is narrated from Sayyiduna Abdullah bin Amr bin al-As (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "The time of the Zuhr prayer is when the sun has declined and a man's shadow has become equal to his height, as long as the time of Asr has not come."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1707